Platinum Group Element Depletion Research Articles

Variant Offset-Type Platinum Group Element Reef Mineralization in Basal Olivine Cumulates of the Kapalagulu Intrusion, Western Tanzania

Abstract The Kapalagulu intrusion in eastern Tanzania hosts a major, 420-m-thick, stratiform/stratabound platinum group element (PGE)-bearing sulfide zone—the Lubalisi reef—within a prominent, chromititiferous, harzburgite unit close to its stratigraphic base. Several features of the vertical base and precious metal distributions (in a composite stratigraphic section based upon two deep exploration drill holes) display similarities to those of offset-type PGE reefs that formed under the overall control of Rayleigh fractionation: (1) composite layering (at several scales) defined by systematic vertical variations of sulfide and precious metal contents and intermetallic ratios, indicating repeated cycles of PGE enrichment and depletion in the order Pd-Pt-Au-Cu, and (2) in the lower part of the reef, stratigraphic offsets of the precious metal peaks below peak sulfide (Cu) content. The form and geochemistry of the reef are consistent with overturns of basal liquid layers within a liquid layering system (i.e., stable density-driven stratification of a magma chamber), plus at least two minor inputs of parental magma during which the resident magma was recharged with sulfur and metals, and the effective depletion of precious metals in the magma midway through reef development. The Lubalisi reef differs from classic offset-type PGE reefs, however, principally because individual Pd, Pt, and Au enrichment peaks are coincident, not offset. The reef is set apart from other offset-type PGE reefs in three additional ways: (1) its association with olivine cumulates that crystallized soon after initial magma emplacement and well below the first appearance of cumulus pyroxene or plagioclase (implying attainment of sulfide saturation and precious metal enrichment without prolonged concentration of sulfur and chalcophile metals by normal magma cooling and differentiation), (2) the probable role of chromite crystallization in not only triggering sulfide segregation during reef formation but also facilitating precious metal enrichment in the early stages of reef development, and (3) its great width. The early stage of fractionation may also help explain the coincident precious metal peaks through its effect on apparent precious metal partition coefficients.

Platinum group elements in mantle melts and mantle samples

A large data compilation has been assembled of platinum group element (PGE) analyses in mantle melts and mantle rocks, the latter including an assortment of xenoliths and obducted mantle massifs. The degree of correlation has been investigated among the PGEs and with other major element variables such as Al2O3, TiO2 and Mg number, and the results are considered in the context of the current paradigm for the behaviour of highly siderophile elements in the silicate Earth.Primitive mantle melts have a wide range of PGE contents. Komatiites have the highest abundances of all the PGEs, show the strongest correlations between Pt and Rh, Pt and Pd and between the iridium-group PGEs Ir, Ru and Os (IPGEs). Most basalts of all affinities have lower levels of Pt and Pd and much lower levels of Ir, Ru and Os than komatiites. Within the basalt grouping Rh has stronger affinities with the IPGEs. Picrites and Archaean basalts are intermediate between these two groups. MORBs and a small proportion of continental LIP basalts show strong depletions in all PGEs attributable to retention of sulfide in their mantle source rocks, or sulfide liquid fractionation on ascent. The degree of PGE depletion in other basalts is probably attributable to equilibration with sulfide, but is less than would be expected under conventional models of sulfide extraction, and is instead attributed to mixing of magmas generated at variable depths incorporating both sulfide-saturated and undersaturated components. Basalts with Pt and Pd contents higher than typical komatiites are rare, a notable example being B1-type parent magmas to the Bushveld Complex, which have komatiite-like relative PGE abundances and Pt, Pd and Rh abundances up to a factor of two higher than komatiites for comparable Ti contents.The mantle composition array as a whole is characterized by variable degrees of depletion of Pt, Pd and Rh in Al-poor, melt-depleted harzburgite/dunite lithologies; lack of depletion in these elements in Al-bearing lherzolites; and a lack of systematic variation in IPGEs across this range. Strongest correlations across the entire set are observed between Ir, Ru and Os; and between Pt and Rh. Melt-depleted cratonic mantle samples are notably more deficient in Pd than in Pt, but comparable Pd-enriched components are not represented in the available data from continental environments. The only group of mantle melts that systematically record high Pd/Pt ratios are MORBs; if these are indeed the complement of the depleted cratonic mantle suite then the melt depletion recorded by the cratonic mantle suite occurred at low pressure prior to tectonic underplating of the depleted lithosphere beneath the cratons. A filtered subset of orogenic peridotite compositions that are thought not to have been affected by significant extents of melt extraction or metasomatic refertilization have median concentrations of 3.9ppb Os, 2.9ppb Ir, 6.3ppb Ru, 1.0ppb Rh, 6.2ppb Pt, 5.4ppb Pd, and Cu/Pd ratio of 5500, which we consider to be representative of the modern convecting mantle. The convecting mantle has PGE proportions closely resembling those of lunar impact breccias, diverging considerably from chondritic proportions and attributable to the presence of a late veneer-derived, predominantly sulfide-hosted component. The compositions of mantle peridotites show considerable scatter attributable to the combined effects of measurement error and a strong covariance due to a heterogeneous distribution of sulfide in the small samples typically chosen for pulverization. The intensity of the covariance between all of the PGE due to sampling error gives a false impression of a genetic trend toward highly enriched PGE in some samples which could be mistaken for the effects of metasomatism; however no plausible metasomatic process would be expected to retain the tight interelement correlations shown.

Origin of PGE depletion of Triassic magmatic Cu–Ni sulfide deposits in the central-southern area of Jilin province, NE China

Triassic mafic–ultramafic intrusions are widespread in the central-southern part of Jilin province, northeast China. Many of them are ore-bearing intrusions of considerable economic value. They compose a number of magmatic Cu–Ni sulfide deposits which are characterized by depletion of platinum-group elements (PGEs). In this study, an attempt was made to clarify the relationship between the PGE content of the mafic–ultramafic magmas and the nature and the evolution of their mantle sources. Precise PGE data of some of the typical ore-bearing intrusions are reported and the factors which may affect the PGE content of the intrusions are discussed, such as the presence of PGE in the mantle, the degree of partial melting of the mantle and the evolution of the mantle. In the course of the evolution of the mantle, a large proportion of PGE which are present as fusible components such as sulfides will be extracted, and most of the refractory PGE components such as alloys will be retained in mantle residues. Without further replenishing of PGE, later melts of a residue will contain less PGE. It is because the Triassic mafic–ultramafic intrusions are derived from PGE-depleted mantle that the magmatic Cu–Ni sulfide deposits in the central-southern part of Jilin province are reduced in PGE. We conclude that most of the young mafic–ultramafic intrusions are probably poor in PGEs.

Maggie Hays Ni Deposit: Part 2. Nickel Mineralization and the Spatial Distribution of PGE Ore-Forming Signatures in the Maggie Hays Ni System, Lake Johnston Greenstone Belt, Western Australia

The Maggie Hays Ni-Cu sulfide deposit, located in the ~2.9 Ga Lake Johnston greenstone belt of Western Australia, is hosted in a peridotitic to dunitic subvolcanic feeder that cuts felsic volcanic rocks and sedimentary units. Ore formation at Maggie Hays is attributed to assimilation of a sulfide-rich metasedimentary unit located above the subvolcanic feeder, which induced sulfide saturation within the feeder magmas. Massive and disseminated sulfides display chalcophile element fractionation trends indicative of in situ sulfide crystallization. Sulfide saturation and ore formation within the system generated enriched and depleted chalcophile elements ore-forming signatures in the silicate magmas. These ore-forming signatures are quantified as deviations from calculated background abundances and are spatially constrained to known mineralization within the system using a three-dimensional deposit model. Platinum group element (PGE) depletion and enrichment signatures occur approximately 300 m upstream from mineralization at the intersection of the feeder conduit and sulfide-rich sedimentary unit, interpreted to mark the point of sulfide saturation within the system. The magnitude of PGE enrichment increases with proximity to mineralization. PGE depletion signatures exhibit a more complex pattern, attributed to mixing between sulfide liquid, depleted silicate magma, and undepleted recharging magma. The study of the Maggie Hays mineralization and its host sequence demonstrates that enrichment and depletion of the highly chalcophile PGEs, relative to abundances expected in sulfide-undersaturated mantle-derived magmas, is a powerful exploration tool. Constraining the spatial distribution of enrichment and depletion signatures in the context of a robust volcanic, stratigraphic, magmatic, and metallogenic model allows for quantification of the size of Ni sulfide-forming systems and targeting of Fe-Ni-Cu mineralization in komatiite-bearing sequences.

Platinum-group elements in the oxide layers of the Hongge mafic–ultramafic intrusion, Emeishan Large Igneous Province, SW China

The Hongge layered intrusion (259.3±1.3Ma) is one of several mafic–ultramafic intrusions that host giant Fe–Ti–V oxide ore deposits in the ~260Ma Emeishan Large Igneous Province (ELIP), SW China. The Hongge intrusion consists of a lower olivine clinopyroxenite zone (LOZ), a middle clinopyroxenite zone (MCZ) and an upper gabbro zone (UGZ). Most of the 14 to 84m-thick and 300 to 1700m-long economic Fe–Ti–V oxide ore layers occur within the MCZ. This paper reports the concentrations of PGE in the oxide layers of the Hongge intrusion. Unlike in the economic PGE (platinum-group elements) mineralized (up to 1.2ppm Pt and 1.8ppm Pd) coeval Xinjie intrusion (259±3Ma), the oxide layers in the Hongge intrusion contain very low PGE (total PGE: 0.09–63.5ppb). Chromite-bearing horizons in the Hongge intrusion are enriched in IPGE (Ir: 0.46–0.65ppb; Ru: 2.25–3.29ppb) relative to PPGE (Pt: 0.54–1.28ppb; Pd: 0.30–0.90ppb). In contrast, the massive magnetite layers in the Hongge intrusion show no IPGE enrichments relative to PPGE. All our samples from the Hongge intrusion collectively show no correlation between PGE and S contents, and weak positive correlations between IPGE and Cr contents, indicating removal of Ir, Ru from magma with crystallization of chromite. Positive correlations exist between IPGE and PPGE in the samples, indicating that all of the PGE was controlled by sulfide liquid. Most of the samples have mantle-normalized PGE patterns with a slope similar to that of the Emeishan picritic basalts, which are less fractionated than the coeval high-Ti basalts derived from the same Fe-, Ti-, V-rich magma series. The Hongge oxide-rich samples are characterized by Cu/Pd ratios (1.47–202×104) significantly higher than those for primitive mantle and the coeval picrites (0.99×104). The PGE tenors in bulk sulfides (i.e., in recalculated 100% sulfides) in the sulfide-bearing oxide ores of the Hongge intrusion (<0.1–3ppm) are 2–3 orders of magnitude lower than the Xinjie intrusion (10–100ppm). This, together with extremely high Cu/Pd ratios in both sulfide-bearing and sulfide-barren oxide-rich samples, indicates that the parental magma of the Hongge intrusion was depleted in PGE. We suggest that PGE depletion in the Hongge parental magma was due to previous sulfide segregation at depth, mainly due to crustal contamination. The PGE-depleted sulfides in the Hongge intrusion are thought to have formed by second-stage sulfide saturation and segregation due to fractional crystallization involving abundant magnetite after magma emplacement at Hongge.

Platinum group element and nickel sulphide ore tenors of the Mount Keith nickel deposit, Yilgarn Craton, Australia

A set of platinum group element (PGE) analyses of about 120 samples from a 250-m continuous drill core through the Mount Keith komatiite-hosted nickel orebody, combined with Ni, Cu, Co, S, and major elements, reveals a complex trend of covariance between the original cumulus components of a thick sequence of nearly pure olivine–sulphide liquid adcumulates. The intersection is divided into informal chemostratigraphic zones, defined primarily by combinations of fine-scale cyclicity in original olivine composition, defined by Mg#, and sulphide composition, defined by Pt/S and Ni/S. Contents of Ni and PGE in 100% sulphides (tenors) were determined from linear regressions of the Ni–S and PGE–S covariance for each zone. Inferred olivine compositions range from about Fo92 to Fo94.6 and show a broad decrease from bottom to top of the sequence complicated by numerous reversals, revealing crystallisation in an open conduit system. Ni and PGE tenors of Mount Keith sulphide ores have typical values similar to the type I deposits of the Kambalda Dome. Mobility of S, at least on the scale of 2-m sample composites, is evidently relatively minor. Tenors for the various zones range 12–22% Ni, 370–1540 ppb Pt, 970–3670 ppb Pd, 100–460 ppb Ir, 170–460 ppb Rh, and 710–1260 ppb Ru. Pt, Pd, and Rh tenors are very strongly correlated, but the iridium group of platinum group elements (IPGEs; Ir and Ru) less so. Tenor variations are predominantly controlled by variations in magma/sulphide ratio R (100–350), with a minor component of variance from equilibrium crystallisation trends in the parent magma. PGE depletion in the silicate melt due to sulphide liquid extraction is limited by entrainment of sulphide liquid droplets and continuous equilibration with the transporting silicate magma. Ratios of the PGEs to one another are similar to those in the host komatiite magma, with the exception of Pt, which is systematically depleted in ores, relative to Rh and Pd and relative to host magma, by a consistent factor of about 2 to 2.5. This anomalous Pt depletion relative to PGE element ratios in unmineralized komatiitic rocks matches that observed in bulk compositions of many komatiite-hosted orebodies. The highly consistent nature of this depletion, and particularly the very strong correlation between Pt, Pd, and Rh in the Mount Keith deposit, argue that this depletion is a primary magmatic signal and not an artefact of alteration. Differential diffusion rates between Pt and the other PGEs, giving rise to a low effective partition coefficient for Pt into sulphide liquid, is advanced as a possible but not definitive explanation.

S saturation history of Nain Plutonic Suite mafic intrusions: origin of the Voisey’s Bay Ni–Cu–Co sulfide deposit, Labrador, Canada

The Voisey’s Bay deposit is hosted in a 1.34-Ga intrusion composed of troctolite, olivine gabbro, and ferrogabbro. The sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites, and also occurs as veins along structures in adjacent paragneiss. A dyke is connected to the base of the north wall of the Eastern Deeps Intrusion, and the entry point of this dyke into the chamber is the locus of the Eastern Deeps nickel sulfide deposit. A detailed exploration in the area between the Eastern Deeps and the Ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke. The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized variable-textured troctolite (VTT) that reaches a maximum thickness above the axis of the Eastern Deeps Deposit along the northern wall of the Eastern Deeps Intrusion. The massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the VTT, and there is a marked break in Ni tenor of sulfide between the two. Sulfides hosted in the dyke tend to have low metal tenors ([Ni]100 = 2.5–3.5%), sulfides in Eastern Deeps massive and breccia ores have intermediate Ni tenors ([Ni]100 = 3.5–4%), and disseminated sulfides in overlying rocks have high Ni tenors ([Ni]100 = 4–8%). Four principal processes control the compositions of the Voisey’s Bay sulfides. Coarse-grained loop-textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the Pd/Ir ratio which is due to mineralogical variations where pentlandite is enriched in Pd and Ir is dispersed throughout the mineral assemblage. A decrease in Ir and Rh from the margin of the Ovoid toward cubanite-rich parts at the central part of the Ovoid is consistent with fractionation of the sulfide from the margins toward the center of the Ovoid. The Ovoid ores have higher Ni and Pd tenor than the Eastern Deeps massive sulfides; this is consistent with both a higher R factor and greater degree of silicate parental magma evolution in the Ovoid than the Eastern Deeps. The disseminated sulfides surrounding the Eastern Deeps deposit have some of the highest Ni and Pd tenors at Voisey’s Bay, which are indicative of not only more primitive magmas but also higher R factors than the Ovoid or the Eastern Deeps. VTT and normal-textured troctolite of the Eastern Deeps that contain trace sulfide have 0.1–3 ppb Pt and 0.1–3 ppb Pd, whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of Pt and Pd. Troctolites and olivine gabbros from other parts of the Voisey’s Bay Intrusion and other Nain Plutonic Suite Intrusions, including the Kiglapait, Newark Bay, Barth Island, Mushua, and Nain Bay South Intrusion, also have low platinum group element abundances. Although it is possible that this is a signature of a widespread sulfide saturation event that pre-dated ore formation at Voisey’s Bay, it is more likely that platinum group element (PGE) depletion is a product of the source melting process where low degrees of melting resulted in the retention of PGE in the mantle source. If so, this indicates that PGE depletion should be used with caution as an exploration tool in the Nain Plutonic Suite.

The Kalatongke magmatic Ni–Cu deposits in the Central Asian Orogenic Belt, NW China: product of slab window magmatism?

The Permian Kalatongke Ni–Cu deposits in the Central Asian Orogenic Belt are among the most important Ni–Cu deposits in northern Xinjiang, western China. The deposits are hosted by three small mafic intrusions comprising mainly norite and diorite. Its tectonic context, petrogenesis, and ore genesis have been highly contested. In this paper, we present a new model involving slab window magmatism for the Kalatongke intrusions. The origin of the associated sulfide ores is explained in the context of this new model. Minor amounts of olivine in the intrusions have Fo contents varying between 71 and 81.5 mol%, which are similar to the predicted values for olivine crystallizing from coeval basalts in the region. Analytic modeling based on major element concentrations suggests that the parental magma of the Kalatongke intrusions and the coeval basalts represent fractionated liquids produced by ∼15% of olivine crystallization from a primary magma, itself produced by 7–8% partial melting of depleted mantle peridotite. Positive e Nd values (+4 to +10) and significant negative Nb anomalies for both intrusive and extrusive rocks can be explained by the mixing of magma derived from depleted mantle with 6–18% of a partial melt derived from the lower part of a juvenile arc crust with a composition similar to coeval A-type granites in the region, plus up to 10% contamination with the upper continental crust. Our model suggests that a slab window was created due to slab break-off during a transition from oceanic subduction to arc–arc or arc–continent collision in the region in the Early Permian. Decompression melting in the upwelling oceanic asthenosphere produced the primary magma. When this magma ascended to pond in the lower parts of a juvenile arc crust, it underwent olivine crystallization and at the same time triggered partial melting of the arc crust. Mixing between these two magmas followed by contamination with the upper crust after the magma ascended to higher crustal levels formed the parental magma of the Kalatongke intrusions. The parental magma of the Kalatongke intrusions was saturated with sulfide upon arrival primarily due to olivine fractional crystallization and selective assimilation of crustal sulfur. Sulfide mineralization in the Kalatongke intrusions can be explained by accumulation of immiscible sulfide droplets by flow differentiation, gravitational settling, and downward percolation which operated in different parts of the intrusions. Platinum-group element (PGE) depletion in the bulk sulfide ores of the Kalatongke deposits was due to depletion in the parental magma which in turn was likely due to depletion in the primary magma. PGE depletion in the primary magma can be explained by a relatively low degree of partial melting of the mantle and retention of coexisting sulfide liquid in the mantle.

Rhenium–osmium isotope and platinum-group elements in the Xinjie layered intrusion, SW China: Implications for source mantle composition, mantle evolution, PGE fractionation and mineralization

The Xinjie mafic–ultramafic layered intrusion in the Emeishan large igneous province (ELIP) hosts Cu–Ni–platinum group element (PGE) sulfide ore layers within the lower part and Fe–Ti–V oxide-bearing horizons within the middle part. The major magmatic Cu–Ni–PGE sulfide ores and spatially associated cumulate rocks are examined for their PGE contents and Re–Os isotopic systematics. The samples yielded a Re–Os isochron with an age of 262 ± 27 Ma and an initial 187Os/ 188Os of 0.12460 ± 0.00011 ( γ Os( t) = −0.5 ± 0.1). The age is in good agreement with the previously reported U–Pb zircon age, indicating that the Re–Os system remained closed for most samples since the intrusion emplacement. They have near-chondritic γ Os( t) values ranging from −0.7 to −0.2, similar to those of the Lijiang picrites and Song Da komatiites. Exceptionally, two samples from the roof zone and one from upper sequence exhibit radiogenic γ Os( t) values (+0.6 to +8.6), showing minor contamination by the overlying Emeishan basalts. The PGE-rich ores contain relatively high PGE and small amounts of sulfides (generally less than 2%) and the abundance of Cu and PGE correlate well with S, implying that the distribution of these elements is controlled by the segregation and accumulation of a sulfide liquid. Some ore samples are poor in S (mostly <800 ppm), which may due to late-stage S loss caused by the dissolution of FeS from pre-existing sulfides through their interaction with sulfide-unsaturated flowing magma. The combined study shows that the Xinjie intrusion may be derived from ferropicritic magmas. The sharp reversals in Mg#, Cr/FeO T and Cr/TiO 2 ratios immediately below Units 2–4, together with high Cu/Zr ratios decreasing from each PGE ore layer within these cyclic units, are consistent with multiple magma replenishment episodes. The sulfides in the cumulate rocks show little evidence of PGE depletion with height and thus appear to have segregated from successive inputs of fertile magma. This suggests that the Xinjie intrusion crystallized from in an open magma system, e.g., a magma conduit. The compositions of the disseminated sulfides in most samples can be modeled by applying an R factor (silicate–sulfide mass ratio) of between 1000 and 8000, indicating the segregation of only small amounts of sulfide liquid in the parental ferropicritic magmas. Thus, continuous mixing between primitive ferropicritic magma and differentiated resident magma could lead to crystallization of chromite, Cr-bearing magnetite and subsequently abundant Fe–Ti oxides, thereby the segregation of PGE-rich Cu-sulfide. When considered in the light of previous studies on plume-derived komatiites and picrites worldwide, the close-to-chondritic Os isotopic composition for most Xinjie samples, Lijiang picrites and Song Da komatiites suggest that the ferropicritic magma in the ELIP were generated from a plume. This comprised recycled Neoproterozic oceanic lithosphere, including depleted peridotite mantle embedded with geochemically enriched domains. The ascending magmas thereafter interacted with minor (possibly <10%) subducted/altered oceanic crust. This comparison suggests that the komatiitic melts in the ELIP originated from a greater-than normal degree of melting of incompatible trace element depleted, refractory mantle components in the plume source.

Platinum Group Element Geochemistry of Mineralized and Nonmineralized Komatiites and Basalts

Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new high-precision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores. Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion, and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion, commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiite-hosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within ore-bearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors. Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes.

Progressive mixing of meteoritic veneer into the early Earth’s deep mantle

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old (from the Archaean era), that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1. In particular, the platinum group element (PGE) contents of komatiites provide a unique source of information on core formation, mantle differentiation and possibly core–mantle interaction2, 3, 4, 5, 6, 7, 8. Most of the available PGE data on komatiites are from late Archaean (~2.7–2.9 Gyr old) or early Proterozoic (2.0–2.5 Gyr old) samples. Here we show that most early Archaean (3.5–3.2 Gyr old) komatiites from the Barberton greenstone belt of South Africa and the Pilbara craton of Western Australia are depleted in PGE relative to late Archaean and younger komatiites. Early Archaean komatiites record a signal of PGE depletion in the lower mantle, resulting from core formation. This signal diminishes with time owing to progressive mixing-in to the deep mantle of PGE-enriched cosmic material that the Earth accreted as the ‘late veneer’ during the Early Archaean (4.5–3.8 Gyr ago) meteorite bombardment.

Siderophile and chalcophile metal variations in Tertiary picrites and basalts from West Greenland with implications for the sulphide saturation history of continental flood basalt magmas

Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ∼1,000 m of picritic lavas and discrete 10- to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaigat and Maligât Formations of which the former includes the Naujanguit member, which consists of picrites with 7–29 wt% MgO, 80–1,400 ppm Ni, 5.7–9.4 ppb Pt and 4.2–12.9 ppb Pd. The Naujanguit member contains two horizons of contaminated basalts, the Asuk and Kuganguaq, which have elevated SiO2 (52–58 wt%) and low to moderate MgO (7.5–12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16–0.63 ppb) and Pd (0.13–0.68 ppb) abundances, and in the case of the Asuk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril’sk region. These basalts belong to the 0- to 500-m thick, ∼5,000- to 10,000-km3 Nadezhdinsky Formation, which is centered in the Noril’sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metal-depleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10–20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N–S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril’sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril’sk system.

Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation

A well-developed, 1,000 m thick basaltic sequence in the Yangliuping region, northern part of the Emeishan basalt province, includes the Lower and Middle Units of tholeiitic basalts and an Upper Unit of both tholeiites and subalkalic basalts. The basalts contain 42–55 wt% SiO2 and 4.1–8.3 wt% MgO. Most of these lavas have Gd/Yb > 2.0, Zr/Nb < 12, and ɛNd(260 Ma) values from +2.5 to +4.7. The platinum-group elements (PGE) are very mildly depleted in most of the basalts which contain 8–19 ppb Pt and 7–27 ppb Pd. However, a significant proportion of the Middle Unit basalts are strongly depleted in PGE with some samples having concentrations lower than detection limits. They have extremely high Zr/Nb ratios (up to 14.5) and low ɛNd(260 Ma) values (+3.21 to +0.65), features of extensive lower crustal contamination. Some samples in this unit have high Ni/Pd (3,965–61,198) and low Pd/Cr (410,000–3,930,000) ratios, indicating sulfide segregation and PGE depletion prior to eruption. The primary magmas were S-undersaturated and derived from partial melting at variable depths in the upper mantle. The early and late stage magmas, as represented by the Lower and Upper Units, underwent AFC processes which induced mild S-saturation and PGE depletion in some of the basalts, whereas the magmas represented by the Middle Unit experienced more extensive crustal contamination resulting in stronger S-saturation and in most cases significant PGE depletion.

Siderophile and Chalcophile Metal Variations in Flood Basalts from the Siberian Trap, Noril'sk Region: Implications for the Origin of the Ni-Cu-PGE Sulfide Ores

The Talnakh, Kharaelakh, and Noril’sk I intrusions of the Noril’sk region are coeval with the Permo-Triassic Siberian trap flood basalt and contain one of the largest known resources of Ni-, Cu-, and platinum group element (PGE)-enriched sulfide mineralization. The ~3.5-km-thick stratigraphy of the basalts consists of a Lower sequence of high Ti alkalic, subalkalic, and picritic basalts, and an Upper sequence of low Ti basalts. The stratigraphy of the Upper sequence consists of a group of picritic and tholeiitic basalts assigned to the Tuklonsky Formation, overlain by the Nadezhdinsky Formation which is represented by a sequence of contaminated tholeiites that show an upward decline in the degree of contamination into the overlying Morongovsky and Mokulaevasky Formations. The Nadezhdinsky Formation is depleted in Ni, Cu, and PGE, with the most depleted basalts having concentrations below the analytical determination limits of 0.06 ppb for Pd and 0.1 ppb for Pt and a factor of at least 100 less than the Pd and Pt concentrations of the rest of the Upper sequence. The greatest PGE depletion occurs a short distance above the base of the Nadezhdinsky, and these flows are overlain by basalts that exhibit a gradual recovery in PGE concentration toward values typical of the Mokulaevsky Formation and the overlying basalts (~6.8–12.1 ppb Pt and ~7.2–16.5 ppb Pd). Within the Nadezhdinsky Formation, there is a strong correlation between Pd/Zr and La/Sm ratios that indicates that the magmas with the largest contribution from crustal material are also the most PGE depleted. Moreover, the contribution of crust to the magma and the degree of metal depletion decreased through time. The PGE-undepleted Upper sequence tholeiites have Pd/Pt of 0.45 to 2.0, and the ore-forming sulfides (Pd/Pt ~3–4) were probably formed from magmas with Pd/Pt ratios at the upper limit of this range; the metal-depleted basalts have lower values (Pd/Pt ~0.3). The high Pd/Pt ratio of the ores and the low Pd/Pt ratio of the basalts indicate that the processes of ore formation and flood basalt magmatism were important because they indicate that the basalts have the Pd/Pt ratio of a magma that has segregated sulfide with the same Pd/Pt ratio as the Noril’sk ores. Moreover, the ore deposits are located in the region where the metal-depleted Nadezhdinsky Formation is ~500 m thick and forms a >5,000-km3 volcanic center. The continuous changes in basalt chemistry through the stratigraphy of the Upper sequence are consistent with processes that took place in a very large staging chamber rather than within multiple ~5- to 10-km3 discrete high-level magma chambers on the scale of the mineralized intrusions. In the staging chamber, Tuklonksy-type magmas interacted with crust to produce the contaminated basalts of the Nadezhdinsky Formation. The staging chamber containing the crustally contaminated magma was then replenished with PGE-undepleted magma. Initially, the replenishing magma became sulfide saturated, thereby producing the ore-forming sulfides, and the magma in the staging chamber remained sulfide-saturated. As further injections of PGE-undepleted magma entered the staging chamber, the melt became sulfide undersaturated and magmas tapped from it were progressively less PGE depleted. The deposits of the Noril’sk, Talnakh, and Kharaelakh intrusions were formed by injection of olivine-bearing melts containing immiscible sulfide that were produced in the staging chamber. The ores inherited a heavy sulfide isotope composition possibly by reaction with evaporite-laden country rocks. The unradiogenic composition of the Os in the ores is consistent with interaction between the crustally contaminated magma (which had become depleted in the PGE and radiogenic Os) in the staging chamber with later pulses of uncontaminated mantle-derived magma. The high PGE contents of PGE-undepleted Siberian trap are due to interaction of plume-generated picritic magmas with lithosphere that had been depleted in sulfide by previous melt extraction.

Komatiites and nickel sulfide ores of the Black Swan area, Yilgarn Craton, Western Australia. 4. Platinum group element distribution in the ores, and genetic implications

The Black Swan komatiite sequence, in the Eastern Goldfields province of the Archaean Yilgarn Craton in Western Australia, is a body of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. The massive sulfide orebodies of the Black Swan Succession are pervasively depleted in all platinum group elements (PGEs), particularly Pt and Pd, despite very high Ni contents. This depletion cannot be explained by R-factor variations, which would also require relatively low Ni tenors. The PGE depletion could be explained in part if the ores are enriched in a monosulfide solid solution (MSS) cumulate component, but requires some additional fractional segregation of sulfide melt upstream from the site of deposition. The Silver Swan orebody shows a remarkably consistent vertical zonation in PGE contents, particularly in Ir, Ru, Rh, Os, which increase systematically from very low levels at the stratigraphic base of the sulfide body to maxima corresponding roughly with the top of a lower layer of the orebody rich in silicate inclusions. Platinum shows the opposite trend, but is somewhat modified by remobilisation during talc carbonate alteration. A similar pattern is also observed in the adjacent White Swan orebody. This zonation is interpreted and modelled as the result of fractional crystallisation of MSS from the molten sulfide pool. The strong IPGE depletion towards the base of the orebody may be a consequence of sulfide liquid crystallisation in an inverted thermal gradient, between a thin rapidly cooling upper rind of komatiite lava and a hot substrate.

The concentrations of the noble metals in Southern African flood-type basalts and MORB: implications for petrogenesis and magmatic sulphide exploration

Concentrations of the platinum-group elements have been determined in several suites of southern African flood-type basalts and mid-ocean ridge basalt (MORB), covering some 3 Ga of geologic evolution and including the Etendeka, Karoo, Soutpansberg, Machadodorp, Hekpoort, Ventersdorp and Dominion magmas. The magmas cover a compositional range from 3.7 to 18.7% MgO, 26–720 ppm Ni, 16–250 ppm Cu, and <1–255 ppb total platinum-group elements (PGE). The younger basalts (Etendeka, Karoo) tend to be depleted in PGE relative to Cu, while most of the older basalts (Hekpoort, Machadodorp, Ventersdorp, Dominion) show no PGE depletion relative to Cu. Further, the younger basalts tend to have lower average Pt/Pd ratios than the older basalts, and the MORBs have lower average Pt/Pd than the continental basalts within the broad groupings of "old" and "young" basalts. This may reflect (1) a decreasing degree of mantle melting through geologic time, and (2) source heterogeneity, in that the MORBs are derived from predominantly asthenospheric mantle, whereas the continental basalts also contain a lithospheric mantle component enriched in Pt. In addition to these factors, some PGE fractionation also occurred during differentiation of the magmas, with Pd showing incompatible behaviour and the other PGE variably compatible behaviour. The examined southern African flood-type basalts and MORB appear to offer limited prospects for magmatic sulfide ores, largely because they show little evidence for significant chalcophile metal depletion that could be the result of sulphide extraction during ascent and crystallization.

Detection of noble metal depletion in layered mafic intrusions: a potential aid to exploration for platinum-group element deposits

In layered mafic intrusions, economically important platinum-group element (PGE) mineralisation can result from collection of PGE and Au from the silicate melt by Ni ( ± Cu) S droplets, which settle to form PGE- and Au-enriched layers. An important consequence is that the PGE and Au are depleted in the remaining silicate fraction. The ability to recognise PGE and Au depletion in rocks crystallised from a depleted melt would be useful in exploration for discriminating between favourable (containing PGE- and Au-depleted silicates) and unfavourable layers (those that had not experienced NiS collection of PGE and Au). Drychlorination was applied to drill core samples of the Fox River Sill (Manitoba, Canada) in order to remove metallic minerals. The remaining silicate fractions were digested by microwave heating in a mixture of HFHClHNO 3, evaporated to near dryness, re-digested in aqua regia, and re-constituted in distilled de-ionized water. The solutions were analysed by ICP-MS for PGE and Au. The PGE and Au contents of the silicate fractions of samples collected from mineralised layers were significantly depleted relative to the PGE and Au contents of silicate fractions of samples collected from a non-mineralised layer. Verification of the ICP-MS method for the determination of Pd was carried out by means of DCP-AES. Instrumental neutron activation analysis (INAA) was used to verify the method for Au and Ir.